Specular Color

[Alundra]

I searched on the shader code of UnrealEngine to try to understand and I think I found the answer.
You can see in BasePassForForwardShadingPixelShader :

half DielectricSpecular = 0.08 * Specular;
half3 DiffuseColor = BaseColor - BaseColor * Metallic; // 1 mad
half3 SpecularColor = (DielectricSpecular - DielectricSpecular * Metallic) + BaseColor * Metallic; // 2 mad

Looks like the answer of specular and metallic values.
All that help to understand all the PBR (physical material).
Since Specular is 0.5 as default :
DielectricSpecular = 0.08*0.5 = 0.04.
for Metallic = 0 the calcule end to :
SpecularColor = 0.04.